This invention relates to displays having improved viewability.
Displays may be transmissive or reflective. In a transmissive display, the display lies between the viewer and the light source. In a reflective display, the viewer and the light source are on the same side of the display. Many displays are liquid crystal displays, in which the element which transitions between one optical state and a second optical state in response to an input (e.g., an electrical signal) comprises liquid crystal material.
A preferred type of liquid crystal display employs encapsulated liquid crystal material, in which liquid crystals are encapsulated or dispersed in a matrix (or containment medium) which can be, e.g., a polymer. When a voltage corresponding to a sufficiently strong electric field is applied across the encapsulated liquid crystal material (the "field-on" condition), the alignment of the liquid crystals is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystals is random and/or influenced by the liquid crystal-matrix interface, so that the liquid crystal material scatters incident light. The applied voltage at which the liquid crystal material begins to change from its field-off condition to its field-on condition is called the threshold voltage. If a reflector is positioned behind the display, then a reflective display is obtained, which appears bright in the field-on condition and darker in the field-off condition. If a light source is positioned behind the display, then a transmissive display can be obtained.
Encapsulated liquid crystal displays can include a pleochroic dye in the liquid crystal material to provide light control capabilities through absorption. In the field-on condition, the alignment of the pleochroic dye is determined by the alignment of the liquid crystals (which in turn is determined by the electric field). In this alignment, the absorption of incident light by the dye is at a minimum or substantially reduced, so that a substantial amount of incident light is transmitted. In the field-off condition, the alignment of the pleochroic dye also conforms to the alignment of the liquid crystals (but which are now random or distorted), so that significant light absorption occurs.
Thus, either a reflective or transmissive encapsulated liquid crystal display can be made to appear darker in the field-off condition by the scattering or the absorption of the incident light, or both, and brighter in the field-on condition because scattering and/or absorption is reduced, permitting the incident light to reach the reflector or be transmitted through the display, as applicable.
In reflective displays the reflector can have an important effect on the perceived brightness. At one end of the scale the reflector can be Lambertian, with excellent viewing angle, but low brightness. At the other end of the spectrum would be a specular mirror with viewing angle limited by the surrounding lighting fixtures, but with the brightness of those fixtures.
In a reflective display of the type used in laptop computers, and in particular colored ones, some aperturing of the picture is unavoidable due to the pixel structure. With a Lambertian reflector this leads to excessive light loss. For a specular reflective display obtaining light uniformity and good viewing angles is difficult (e.g., the viewer sees his own reflection in the display). Empirically, some degree of diffusion is needed to produce a pleasing display.
We have invented a display with improved viewability by providing for some residual scattering in the field-on condition.